The retino-geniculo-cortical pathway is considered the primary afferent pathway that subserves visual perception in most mammals. Recent evidence suggests that an important role of the lateral geniculate nucleus (LGN) in this pathway is to modulate or gate the transmission of visual information to visual cortex. This gain control is thought to reflect different behavioral states such as arousal, attention and eye movements. One source of this state-dependent influence is the brainstem. Three approaches are proposed to describe the anatomical and functional connections between the brainstem and LGN, and thus, elucidate the physiological basis of the state-dependent influences on visual perception. The first aim is to describe the light microscopic morphology of individual brainstem axons that project to the LGN. This will be accomplished by injecting, in separate experiments, the new anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L) into six specific cell groups in the brainstem (parabrachial nucleus, locus coeruleus, dorsal raphe nucleus, superior colliculus, parabigeminal nucleus and the nucleus of the optic tract) and serially reconstructing PHA-L labelled processes as they course through the ipsilateral LGN. This technique provides a far superior visualization of the terminal arborizations from thin axons than any previous anterograde tracing method. The second aim is to extend the PHA-L technique to the electron microscopic level. In particular, we will clarify issues about the circuitry through which individual brainstem axons influence LGN neurons. The third aim is to examine the functional consequences of these brainstem-LGN connections. This will be accomplished by observing the effects of electrical or chemical stimulation in the brainstem on the extracellular visual responses of LGN cells to grating and spot stimuli. Plans are described to analyze the effects of stimulation in three brainstem sites (parabrachial nucleus, locus coeruleus, and dorsal raphe nucleus) on the responses of the different functional cell types in the LGN (e.g., normal and lagged X-cells, Y-cells, projections cells, interneurons) and the adjacent perigeniculate nucleus.